Most of the visual content today is still in two dimensional (2D) images or videos which are in form of a sequence of images. Generally, the conventional images and videos do not support changes of viewpoints other than just magnification/scaling or simple shifting. With the advent of stereo or three dimensional display technologies, active shutter and passive polarized eye glasses are now commonly available. More recently, high resolution autostereoscopic displays, which do not require eye glasses, have become available. The input to such autostereoscopic displays is usually i) a video plus a depth map which describes the depth of each pixel in the video or ii) a set of videos at adjacent viewpoints, sometimes called multi-view videos, which are multiplexed on an image frame in a certain format. A lenticular lens or parallax barrier of the autostereoscopic displays perform a spatial filtering so that a user at a certain viewing position will be able to see two different images in his/her left and right eyes, respectively, thus creating a 3D perception.
As noted, depth maps are images (or videos if taken at regular time interval) that record the distances of observable scene points from the optical point of a camera. It provides additional information to the associated color pixels in the color image or video taken at the same position by specifying their depths in the scene. One application of depth maps is to synthesize new views of the scene from the color image or videos (also referred to as texture). Depth maps can also be taken at adjacent spatial locations to form multi-view depth images or videos.
Efficient compression methods for depth map images and videos are therefore important for the efficient storage and transmission of such images. Intra coding plays an important role in hybrid video coding schemes, especially in applications such as effect access, references for prediction, error resilience, bit rate control, low-complexity encoding, etc. See, G. J. Sullivan et al, “Overview of the High Efficiency Video Coding (HEVC) Standard,” IEEE Trans. Circuits and Systems for Video Technology, vol. 22, pp. 1649-1668, (2012); G. Tech et al., “3D-HEVC draft text 1,” in Proceedings of the 5th Meeting of Joint Collaborative Team on 3D Video Coding Extensions (JCT-3V), Document JCT3V-E1001, Vienna, Austria, (August 2013); and J. Lainema et al., “Intra Coding of the HEVC standard,” IEEE Trans. Circuits and Systems for Video Technology, vol. 22, pp. 1792-1801, (2012), all of which are incorporated herein by reference in their entirety. Intra coding is performed relative to information that is contained only within the current frame and not relative to any other frames in the video sequence.
The intra coding of state-of-the-art depth compression algorithms is usually based on spatial sample prediction followed by discrete cosine transform (DCT)-based coding. However, these methods may not be efficient due to the piecewise smooth nature of depth maps. The conventional DCT-based intra coding methods need to use a considerable number of bits to cope with depth discontinuities in depth maps. At high compression ratio, the DCT-based intra coding usually generates artifacts at the discontinuities and degrades the coding quality. Hence, more efficient intra coding for depth compression is in demand.